(1) Field of the Invention
The present invention relates to a two-light flux interference exposure device, a two-light flux interference exposure method, a semiconductor light emitting element manufacturing method, and a semiconductor light emitting element.
(2) Description of Related Art
Light-emitting diodes have widely been put into practical use as semiconductor light emitting elements by p-n junction of a compound semiconductor and employed mainly for purposes of optical transmission, display and special illumination. Recently, a white light-emitting diode using a nitride semiconductor and a phosphor has also been put into practical use. The white light-emitting diode is greatly expected to be developed into use for general lighting in the future. However, since particularly the white light-emitting diode has an insufficient energy conversion efficiency as compared with existing fluorescent lamps, the white light-emitting diode necessitates significant improvement in the efficiency for use as general lighting. A major cause of low energy conversion efficiency is a low efficiency in extraction of light out of a light-emitting diode but not a low luminance efficiency of a semiconductor. A semiconductor generally has a high refractive index ranging from about 2.5 to about 4. Accordingly, when light generated inside a light-emitting layer is externally extracted, about 20% to 30% Fresnel reflection and refraction of a radiation angle occur on a light extraction surface. When a critical angle has been exceeded, the refraction of a radiation angle causes total reflection such that all the light is confined to the inside of the element. The confined light is absorbed into semiconductor layers, electrodes or the like thereby to be attenuated, while repeating reflection in the inside of the element. Consequently, the light changes to heat. General light emitting diodes have a light extraction efficiency of about 20%, and accordingly, 80% light is lost in the inside of the element.
The structure that a semiconductor surface is corrugated is suggested in order that the aforesaid problem may be overcome (see JP-A-2003-86835, for example). When such a corrugated structure is provided on the semiconductor surface at a light-extracting side, the effect of light scatter causes total reflection to disappear, so that the optical transmittance of nearly 50% can be obtained over a wide radiation angle. When it is assumed that a back contact has a 100% reflectance, the light extraction efficiency can be increased nearly 50%. Furthermore, the inventors have confirmed that the light extraction efficiency can be increased to 70 to 80% when a period of the corrugated structure is reduced approximately to an optical wavelength of a light emitting diode. Since the wave nature of light in the corrugated structure is actualized by reducing the period of the corrugated structure approximately to the optical wavelength of the light emitting diode. The light transmittance is improved by a refractive effect. Although different names such as photonic crystal and motheye structure are used depending upon a shape of the periodic corrugated structure, the same principle is employed for improving the light extracting efficiency.
However, it is not easy to make a periodic structure with a wavelength order used in the photonic crystal or motheye structure. Furthermore, there is also a problem that the manufacturing costs are high. For example, in the case of a blue light emitting diode made from a nitride, an optic wavelength needs to be set to 184 nm and a convex portion of the corrugated structure needs to have a width set to about 90 nm when the wavelength of light to be emitted is set to 460 nm and the nitride crystal has a refractive index of 2.5. It is difficult to make a periodic structure having such a size even by a mot-advanced photolithography technique using a current ArF excimer laser. Moreover, it is impossible from the point of costs to use such an extremely expensive manufacturing apparatus for manufacture of photo diodes. No apparatus for manufacture of compound semiconductors with a small substrate diameter is available. Furthermore, an electron photolithography process used for manufacture of current photonic crystal or motheye structure has an extremely low productivity and accordingly cannot be used for manufacture of low cost light emitting diodes.